Recombinant adeno-associated virus (rAAV) has widely been used as a vehicle for the delivery of genetic material to target cells. AAV has several advantages over other viral vectors. For example, wild-type AAV has not been associated with any pathological human condition, rAAV does not contain native viral coding sequence and persistent expression of delivered transgenes has been observed in many applications.
Under normal circumstances, AAV packages a single-stranded DNA molecule of up to 4800 nucleotides in length. Following infection of cells by the virus, the intrinsic molecular machinery of the cell is required for conversion of single-stranded DNA into double stranded form. The double-stranded form is then capable of being transcribed, thereby allowing expression of the delivered gene to commence. It has been shown in a number of cell and tissue types that second strand synthesis of DNA by the host cell is the rate-limiting step in expression. By virtue of already being packaged as a double stranded DNA molecule, self-complementary AAV (scAAV) bypasses this step, thereby greatly reducing the time to onset of gene expression.
Self-complementary AAV is generated through the use of vector plasmid with a mutation in one of the terminal resolution sequences of the AAV virus. This mutation leads to the packaging of a self-complementary, double-stranded DNA molecule covalently linked at one end. Vector genomes are required to be approximately half genome size (2.4 KB) in order to package effectively in the normal AAV capsid. Because of this size limitation, large promoters are unsuitable for use with scAAV. Most broad applications to date have used the cytomegalovirus immediate early promoter (CMV) alone for driving transgene expression. However, it has been shown by others that transgene expression with CMV markedly drops off in certain tissue types, such as eye and liver, sometimes as early as two weeks post-injection. A long acting, ubiquitous promoter of small size would be very useful in a scAAV system.
The chimeric CMV-chicken β-actin promoter (CBA) has been utilized extensively as a promoter that supports expression in a wide variety of cells when in rAAV vectors delivered to retina. In addition to broad tropism, the present inventors have observed that CBA also has the capacity to promote expression for long periods post infection (Acland, G. M. et al. Mol. Ther., 2005, 12(6):1072-1082). CBA is ˜1700 base pairs in length, too large in most cases to be used in conjunction with scAAV to deliver cDNAs (over 300 bps pairs in length).